This invention relates to an ink jet recording method in which a plurality of inks different in color are used to record a color image for every picture element.
In general, an ink jet printer for recording images with ink droplets jetted from the nozzles, being simple in construction, can be miniaturized and manufactured at low cost. In addition, the ink jet printer is advantageous in that images can be recorded in black and white or in color on sheets of ordinary paper such as sheets for papers submitted for instance by students (hereinafter referred to as "paper sheets", when applicable) or copying sheets.
A conventional ink jet printer is designed as follows: That is, the ink jet printer has an ink jetting energy producing unit comprising piezo-electric elements or electrothermic conversion elements as an ink jetting source. The ink jetting source is operated to jet ink droplets from the nozzles. As shown in FIG. 13, the ink droplets 101 thus jetted (only one ink droplet shown) are stuck onto and absorbed by a recording medium 100 such as a recording sheet provided only for the printer or a sheet of ordinary paper such as a paper sheet or copying sheet, thus recording an image on it. The printing operation is relatively low in noise. In addition, with the ink jet printer, an image can be recorded not only in black and white but also in color without a fixing process.
On the other hand, in the case where it is required for the ink jet printer to record an image in full-color on a recording medium low in ink absorptivity such as a copying sheet generally used with an electrophotographic copying machine or a paper sheet instead of the recording sheet provided only for the printer, the ink jet printer suffers from the following difficulties: If, in this case, the full-color image recording operation is carried out with the ink which dries slowly to record character or line images with high picture quality, then as shown in FIG. 14 the ink droplets 101 stuck on the recording medium 100, not being immediately dried, flow, so that the adjacent ink droplets 101 meet each other. As a result, so-called "color bleeding" occurs between the adjacent ink droplets different in color; that is, colors are unintentionally mixed with each other. Thus, the resultant color image is low in picture quality.
On the other hand, in the case where a copying sheet or paper sheet is employed as the recording medium, and the ink is used which dries and permeates relatively quickly, the color bleeding between the adjacent ink droplets different in color is suppressed. However, as shown in FIG. 15, the ink droplets 101 immediately permeate the recording medium deep, so that no color materials remain on the surface of the sheet. Hence, the regions printed with the ink droplets 101 are low in density, and small in the range of color reproducibility, and the contours of the recorded character or line images are relatively heavy, and feathering occurs, so that the recorded image is considerably low in picture quality.
In order to overcome the above-described problems, the ink jet printer has employed the following method: In the method, the picture quality of the color image takes precedence over the others: that is, the color inks are increased in drying speed to prevent the occurrence of the aforementioned "color bleeding", thereby to improve the picture quality of the color image recorded thereby. More specifically, as disclosed by Japanese Patent Application Publication No. 11781/1987 a surface active agent is added to the inks, or as proposed by the present Applicant a block copolymer of propylene oxide and ethylene oxide is added to the inks (cf. Japanese Patent Application (OPI) No. 325574/1992 (the term "OPI" as used herein means an "unexamined published application")), to increase the permeability of the inks.
In the case where all the color inks used are high in drying speed, as shown in FIG. 15 the inks permeate the sheet more in the direction of depth (thickness) than in the other directions, and accordingly the record on the surface of the sheet is lowered in density, and the character and line images recorded with the black ink is deteriorated in picture quality. Hence, in a color ink jet printer using black ink together with cyan, magenta and yellow inks, in order to record characters with high density, only the black ink is low in permeability.
However, the above-described conventional ink jet printer still suffers from the following problem: That is, when, in the case where the ink jet printer uses color inks high in permeability and black ink low in permeability, the region of an image recorded with an ink high in permeability merges with the region of an image recorded with an ink low in permeability, then as shown in FIG. 14 the latter ink low in permeability diffuses into the former ink high in permeability, so that bleeding occurs between the black ink and the color ink. Thus, the resultant image is low in picture quality.